The invention relates to a control system for a machine and more particularly to a microprocessor-based control system for a compressor where the operation of the compressor is controlled by a control module which processes actual compressor operating parameter value signals received at regular intervals from compressor sensors, and if one or more of the parameter values is not at a predetermined setpoint value, modifies operation of the compressor to obtain the predetermined setpoint value.
Control systems for compressors typically use pneumatically or mechanically actuated devices to control compressor components such as compressor inlet valves for example. Such devices control the position of the inlet valve so that the required volume of fluid is supplied to the compressor.
Compressors and the associated compressor control devices are designed to be operated in a single ambient operating temperature range such as -20 to 115.degree. F. When the compressor is operated in ambient temperatures within the ambient operating temperature range, the compressor operates in an efficient manner and the pneumatic and mechanical compressor control devices usually operate as required. However, when the compressor is operated outside the ambient operating temperature range, such as in extremely cold or hot conditions, the pneumatically and mechanically actuated control devices frequently do not operate as required and the efficiency of the compressor is significantly reduced. Running the compressor at a reduced efficiency, reduces the life of the compressor bearings, increases compressor noise and vibration and can significantly increase the frequency of compressor repairs. Additionally, the useful life of the compressor may be greatly decreased as a result of use of known pneumatically and mechanically actuated compressor control devices.
There are additional shortcomings associated with using pneumatic and mechanical controls in portable compressors. Because pneumatic and mechanical devices are comprised of a large number of discrete component parts and because such devices rely on fluid flow through the devices for efficient operation, the control devices frequently do not operate properly even when the compressor is operated in the designed ambient operating temperature range. The component parts may stick or freeze in cold temperatures near freezing. Also, pneumatically and mechanically actuated compressor control devices have a limited useful life and, over time, component parts wear out and must be repaired or replaced. Not only is reliability of known control devices low, the cost to repair and maintain known control devices can be quite high.
Oil flooded screw compressors rely on injected oil to absorb the heat of compression. However, the oil flow causes an increase in power consumption. Accordingly, it is highly desirable to maintain the oil flow at a level high enough to maintain the operating temperatures within the design range, yet not so high as to cause excessive power consumption. For example, at low ambient temperatures and under partial loading conditions, less oil is required for cooling, so the oil flow can be lowered to reduce the power consumption. Known compressors do not have the capability to control the oil flow for optimum performance.
Also, in conventional compressors, neither the speed of the prime mover nor the position of the compressor inlet valve may be changed independent of the other compressor operating parameter. The inlet valve could not be opened or closed without also increasing or decreasing the prime mover speed. This rigid interrelation between inlet valve position and prime mover speed limits a compressor operator's ability to obtain the desired compressor discharge pressure.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative to thereby overcome one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.